1. Field of the Invention
This invention is related to a high voltage device with constant current source and the manufacturing method thereof,
2. Description of Related Art
A metal-oxide-semiconductor field effect transistor (MOSFET) is a form of integrated circuit presently manufactured. Enhancement and depletion mode MOSFET devices are known. The enhancement mode MOSFET device is used in most cases. The depletion mode MOSFET device is basically composed of three parts, namely source (S), gate (G) and drain (D). An N−channel depletion mode MOSFET is adjacent to the gate surface, between the source and drain which are connected by a shallow doped layer with same polar. When the voltage between the gate and the source is positive, the saturated through current rises with the increase of voltage, which is similar to that of the enhancement mode MOSFET. When the gate and the source have same potential and the drain is imposed with positive voltage, the depletion mode MOSFET firstly passes through the linear zone where the current spikes and then arrives at the saturation region with through constant current. At this moment, the drain voltage is known as saturation voltage. The through current of the drain is related to the depth and concentration of the shallow doped layer, that is, the current increases when the depth and concentration increases. When the voltage between the gate and the source is negative, the channel may be intercepted and the through current is zero. Then the gate voltage is defined as cut-in voltage. However, it is impossible for the gate to intercept the channel current if the channel is extremely deep and with high concentration. For the drain and source current is in a state of constant through current when the gate and source voltages are zero, the depletion mode MOSFET is not utilized as conveniently as the enhancement mode MOSFET, that is why it is not manufactured alone by the industry. The depletion mode MOSFET can be used as a constant current source just because it gets through when the gate voltage is zero and the current is almost placed in the saturation region until avalanche breakdown when the drain voltage increases. If the working voltage (i.e. avalanche voltage) range of the depletion mode MOSFET is enhanced to more than 50V, the depletion mode MOSFET can be used widely as the constant current source for the overvoltage protection of DC load connected to AC supply after rectification and filtration. If the avalanche voltage of the drain of the depletion mode MOSFET is within 15V, it can be used as low voltage constant current source of DC supply, such as the constant current source of a Light Emitting Diode (LED) mining lamp.
In practical application, despite low power consumption, most loads require that the supplied current and voltage supply must be stable within a range and can provide protection for part elements of the load. To solve this problem, a constant current or stabilized voltage supply for the load is typically applied. Meanwhile, it is needed to add overcurrent protection for some elements of the load in the circuit under abnormal condition, which results in more components, a complicated circuit and higher percentage of consumed extra power of the supply.
Now, with the wider application of the LED, the LED lamp for lighting indoors and outdoors becomes more popular. The existing LED drive circuit is necessarily provided with a constant current circuit which has many peripheral devices increase cost even if the LED has stable illumination and low luminance fluctuation. Moreover, compared with the power consumed by the LED, the extra power consumed by the peripheral circuit makes up more percentage. Usually, the loss power accounts for 20-30% of the total power, which causes the LED have less than optimal energy saving.